1. Field of the Invention
This invention is directed to an electrocoating composition having an improved cure that uses a blocked isocyanate crosslinking agent blocked with a non-hazardous and non-air polluting compound.
2. Description of the Prior Art
Coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is well known and an important industrial process used particularly in the manufacture of automobiles and trucks. In the electrodeposition of primers, an article, such as an auto body or truck body or parts thereof, is immersed in an aqueous electrocoating bath of an electrocoating composition that contains an aqueous emulsion of a film forming polymer. The article to be electrocoated acts as an electrode resulting in an electrical current being passed between the article and a counter electrode, which is in contact with the aqueous emulsion, until a coating with the desired thickness is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter electrode is the anode. The article to be coated is immersed and passed through the bath over a period of time (time in the bath controls the thickness of the coating that is deposited) and then the coated article is removed from the bath, rinsed with water and baked to form a primer coating on the article.
The aqueous emulsion of film forming polymer used in cathodic electrocoating process also is well known. The film forming polymer typically is a polyepoxide that has been extended and formed into an adduct having amine groups that are subsequently neutralized with an acid and blended with a crosslinking agent, such as a blocked polyisocyanate crosslinking agent. The aqueous electrocoating bath contains the film forming polymer, a blocked crosslinking agent, a pigment paste or a pigment dispersion, coalescing solvents and other additives.
It would be very desirable to use a blocked isocyanate crosslinking agent which utilizes a non-hazardous, non-polluting blocking agent that does not have to be recovered and recycled or disposed of in some manner. Such blocking agents are known but require the baking temperatures for the deposited electrocoating composition to be increased during curing in order to remove these known blocking agents and thereby activate the isocyanate crosslinking agent. Although it is not a problem to increase baking temperatures for small metal parts that are electrocoated but increasing baking temperatures for auto and truck bodies present a major problem because such increases result in significant increases in energy consumption and in manufacturing costs. It would be desirable to use a blocking agent for the isocyanate crosslinking agent that is non-hazardous and non-air polluting, but yet would allow the auto and truck bodies to be baked at normal temperatures typically used in the automobile and truck manufacturing process.
This invention is directed to an improved aqueous cathodic electrocoating composition and a method for the electrodeposition of this composition; the composition comprises a binder of (a) a cathodically electrodepositable film forming resin and (b) a polyisocyanate crosslinking agent blocked with a compound that is non-hazardous and non-air polluting, such as an alkoxy alkanol or an alkyl alcohol; wherein the improvement comprises the use of 0.1-5.0% by weight, based on the weight of the binder, of a water miscible amine acid zwitterion additive that is the reaction product of an amine acid and an alkylglycidyl ether.